I don't care how much people say that a two tier system is 'equal but different', it won't be. Look at the number of posts on here in which people mention that they went to grammar school, even though it was 50+ years ago.
I agree that the system was/is good for those who went, which was the point of them - to separate people at the age of 11 into 'academic' and otherwise, and educated accordingly. It perpetuated the class system, was skewed in favour of boys, and caused splits in families when siblings were given different life chances. For those who didn't go, the system was less good.
There is already a huge amount of snobbery surrounding education in this country (again, as can be seen on GN every time the subject comes up). Even degrees are divided by which University awarded them, which subject they are and so on, and it is very clear that a lot of people sneer at those who are perceived to have 'lesser' qualifications, even though there is no universal standard to compare one against another - it comes down to snobbery and prejudice.
Doing this to 11 year olds whilst pretending that it is for their own good is cruel, and does not take account of the different rates at which children develop, or of the impossibility of making the entrance exam 'fair', and will doom those children to a lifetime of people looking down on their qualifications, however hard they have worked for them, and regardless of how well they could have performed if they had been given different chances.
For the record, the grammar/secondary modern system had been abolished in my area by the time I was 11, so I didn't take an 11+.